1. Field of the Invention
The present invention relates to a hub structure and more particularly to a bearingless hub structure for rotary-wing aircrafts.
2. Description of the Prior Art
A conventional common hub structure for rotary-wing aircrafts is of an articulated type arranged to allow the flapping motion, lead-lag motion and torsional motion of rotor blades. Recently, bearingless hubs utilizing composite materials composed of fiber reinforced plastics having high fatigue strength has been developed. Generally, these bearingless hubs have a structure comprising a hub body secured to a rotor shaft, and a plurality of flexbeam positioned radially at equal spaces apart on the hub body, each flexbeam having a tip-end secured to a rotor blade. The flexbeam is formed of unidirectional composites whose fiber direction is aligned with a spanwise direction, allowing a flapping motion of a rotor blade through it's vertical deflection, a lead-lag motion through it's horizontal deflection, and a pitch change through it's torsion. Amounts of the vertical deflection and torsion are adjusted to be relatively large, and an amount of the horizontal deflection is adjusted to be relatively small.
In order to control pitch angle of the rotor blade, a tubular pitch housing of high bending and torsional stiffness encloses or is positioned parallel to the flexbeam. The pitch housing has an outer end secured to a rotor blade and an inner end supported on the flexbeam through a spherical bearing. Further, the pitch housing is provided at it's inner end with a pitch horn connected to a swashplate through a pitch link. For a main rotor, a lead-lag damper is also provided for the pitch housing. There have been proposed various approaches for installation of lead-lag damper.
A typical example of aforementioned bearingless hub is disclosed in Japanese Patent Public Disclosure No. 89200/1978. In such hub structure, the flexbeam is unidirectional composites, therefore it is characterized in that it is very strong for tensile and bending loads, but very soft in twisting direction. However, in spite of that characteristics of unidirectional composites, it is difficult to provide angles of torsion corresponding to the pitch changes of the rotor blade. Therefore, it is desirable that the flexbeam should be formed into a thin laminated element of low torsional stiffness. However, if the flexbeam be a thin laminated element, it could not endure bending loads caused during quick starting of the slow rotations because the buckling strength of the thin laminated element is significantly low. Thus, the flexbeams having C or I shaped cross section have often be made. A typical example of such flexbeams is disclosed in Japanese Patent Public Disclosure No. 96197/1978. However, in this structure when a centrifugal force is acting on the rotor blade, the torsional stiffness is increased by a tensile stress effect, so it is necessary to increase the length of the flexbeam, and in some cases, the flexbeam length more than 30% of the rotor radius is required.
The hub mechanism including such long flexbeam is not only disadvantageous in regard to weight and drag but also creates a complicated phenomenon referred as an aeroelastic coupling by interference between the bending deflections of the flapping and lead-lag directions and the torsional deflection. That is, the flexbeam is relatively soft with respect to the flexure in the flapping direction and is relatively stiff with respect to the flexure in the lead-lag direction. When the flexbeam is twisted, the ratio of the bending stiffness for the flapping direction to the bending stiffness for the lead-lag direction varies in accordance with the twisted angles and does not take a constant value. This variation of stiffness results in changes of the natural frequency of the rotor blade, and can cause so-called coupling phenomenons in which the flapping motion, the lead-lag motion and the pitch angle change affect to each other.
Further, another disadvantage of the bearingless hub of this type is that it is difficult to provide the damping for the lead-lag direction. An example of the bearingless hub which overcomes the problem of the lead-lag damping is a bearingless type rotor hub shown in Japanese Patent Application No. 61489/1983 (Japanese Patent Public Disclosure No. 186798/1984). The structure described in above Disclosure has pitch housings, each having a radially outer portion and radially inner portion, which are connected to each other by a pair of bearings or flexible elements mounted on the upper and lower areas respectively, and further connected to each other by a lead-lag damper at the front or the rear in the plane of rotation. In this structure, when the rotor blade makes the lead-lag motion, the pitch housing pivots about said bearings or flexible elements, thereby applying a tensile force or a compressive force on the lead-lag damper which in turn provides the desired damping.
The hub structure described in above Disclosure, in the example utilizing bearing pairs, requires two (2) bearings for connecting the pitch housing portions and two (2) bearings for installing the lead-lag damper. The aims of the bearingless hub is to improve the reliability and maintainability and also to reduce the cost by replacing bearings with flexible members. A fact of requiring four (4) bearings for single pitch housing cancels said primary aims. Also, in the hub structure wherein the inner and outer end portions of the pitch housing connected to each other by a pair of flexible elements instead of a pair of the bearings, the flexible elements must yieldingly be deformed in the lead-lag direction. In order to provide the durability against the deformation, the flexible elements are required to make it thinner in the lead-lag direction and to have a certain degree of length in the radial direction. On the other hand, these flexible elements are required to carry a torsional moment transmitted from the pitch link and a flapping load transmitted from the rotor blade, and to have adequate stiffness as well as the stiffness and strength in the lead-lag direction. These requirements for each directions are contradictory to each other, therefore it is very difficult to design the flexible elements which satisfies these requiements, so many complicated structural problems occur. In addition to such severe problems, the flexible elements are lacking in reliability because if any one of the flexible elements is damaged, only remaining flexible elements could not carry the loads, thereby unavoidably leading to a destructive accident. Further, this structure still requires two bearings for installation of a lead-lag damper.